State-of-the-art printing and graphics systems, whether based upon laser jet technology or upon more recently developed thermal ink jet concepts, are able to convert images initially represented as con-tone images into digital representations which can be produced in print and on visual displays at various resolution levels. Current popular resolution levels typically produce about 300 or 600 full or partial tone dots per inch on paper, plastic, or another selected physical surface of choice. According to a typical digital conversion scheme, a continuous physical image can be restructured in software as a matrix of point values for digital processing after which the physical image is reconstituted and reestablished as a two-dimensional array of full or partial tone dots of ink and pigment or as photons of light on a display screen. If the selected tone of the dot on the paper or on the display is colored "black," then the dot can either be a full-tone black dot, for example, or the absence of such a dot at a given presentation position. Given a suitable resolution and depending, of course, upon the distance of the viewer from the physical image being viewed, this composite graphical representation or image can evoke the impression of a continuous, gray scale image.
Con-tone images are currently capable of digital processing to establish full-tone dot images on paper by physical print operation or on a display apparatus, according to user selection. Image enhancement can further be accomplished by varying the size of the dots placed at particular matrix positions. Additional image enhancements are possible by varying the intensity or the tone fullness of the dots themselves. According to one toning approach, namely half-toning, a considerable level of image enhancement can be accomplished. According to this technique of digital half-toning, gray scale and color tone images can be represented as discrete binary (black and white, or color and white only) information elements in printer memory and information processing systems. The half-toning techniques which have been developed in the prior art aim to ensure the essential maintenance or impression of the original physically viewable con-tone image in digitized form. Simply stated, the original con-tone image is disassembled into a grid or array of picture elements or "pixels," as they have come to be known. The pixels themselves, notably, are referred to frequently by a nickname of their own: "pics." Each pixel (or "pic") can be represented by a corresponding number to indicate a predetermined intensity level between selected outer bounds or limits of darkness and brightness, e.g., between black and white, or for example between an intense selected color or tone level and its complete absence. Typical colors or tones may include cyan, magenta, yellow, and black, for example. Cyan is well known as a particularly distinctive blue-green shade; magenta, on the other hand, evokes the impression of a scarlet raspberry hue. Usefully, cyan and magenta can be combined to create pure blue; yellow and magenta express themselves in combination as red; and cyan combined with yellow produces an effective green.
With any of these colors, it is possible according to the prior art to present selected con-tone images to the viewer or reader in terms of a physical output grid of picture elements, pixels or pies, with each such picture element constituting a small part of the total original con-tone picture image. The con-tone physical image thus produced can be viewed in terms of the intensity of a particular tone which has been selected. An example of such a tone can of course be the color "black." The black tone can specifically be established by producing a 100% grey con-tone intensity effective for producing a solid "black" picture element. Gray tone impressions can be produced with full tone black dots by making a binary representation of portions of a selected image in terms of ones, "1's," and zeros, "0's." Accordingly, "one" values are assigned for all pixels which in con-tone are gray to the extent of at least fifty percent (50%). Any con-tone gray intensity which is less than fifty percent in intensity will accordingly be represented as a "zero" value on the binary scale. As suggested above, the same approach used for gray con-tone scaling can apply to colored con-tone images.
Further, in accordance with currently used half-toning processes, the increased spatial resolution offered by advanced binary printer models now on the market may be traded for lower spatial resolution and increased tonal resolution. In other words, instead of striving to maximize the number of dots per inch, by employing the techniques of con-tone toning, larger pixel sizes (i.e., super-pixels) can be employed while still obtaining the desired level of apparent resolution, by simply varying the tone intensity of the particular dots actually physically printed. Simply stated, a continuous image can be approximated by the use of con-tone imaging techniques with relatively large (i.e., not optimally small) pixel sizes. With black and white pixels, this can be accomplished by printing an aggregate of black and white dots, which at normal viewing distance produce the effect or appearance of continuous gray shades.
In order to enhance images produced by modern half-toning techniques, well-known error diffusion techniques, as expressed by Floyd and Steinberg, can be employed. See, for example, Floyd, R. W. and Steinberg, L.; "An Adaptive Algorithm For Spatial Gray Scale;" SID 75 Digest; Society for Information Display, 1975, at pages 36-37. Also see, Meyer, J. D., Dispoto, G. J., and Mather, L. R.; U.S. Pat. No. 4,680,645, which was granted by the U.S. Patent and Trademark Office on Jul. 14, 1987, under the title, "Multiple Level Error Diffusion." The main reason for the elaborate scheme of division and distribution of error in the Floyd-Steinberg method is to minimize visual artifacts. In particular, Floyd and Steinberg's techniques reduce artifacts by sending 7/16 of the error observed at a particular pixel position being processed to the pixel on the same line which is to be processed next. 1/16 of the same error is sent to the pixel directly below the next pixel to be processed; 5/16 of the error is sent to the pixel directly below the one being processed; and finally 3/16 or the errer is sent to the pixel diagonally below and to the left of the pixel currently being processed. This approach is illustrated by FIG. 1 attached herewith and is explicitly set forth in Table I immediately below.
TABLE I ______________________________________ [Prior Art] "ERROR DIFFUSION UNDER FLOYD & STEINBERG" ______________________________________ PIXEL 1: Processing for this pixel (PIXEL 1) has been completed. PIXEL 2: This pixel (PIXEL 2) is being processed and is subject to a predetermined "ERROR" level. PIXEL 3: This pixel (PIXEL 3) is subject to an ERROR allocation in the amount of 7/16 of the ERROR at PIXEL 2. PIXEL 4: This pixel (PIXEL 4) receives an allocation of ERROR in the amount of 3/16. PIXEL 5: This pixel (PIXEL 5) receives an allocation of ERROR in the amount of 5/16. PIXEL 6: This pixel (PIXEL 6) receives an allocation of ERROR in the amount of 1/16. ______________________________________
According to this approach to implementing Floyd & Steinberg, the spatial resolution of the input image being processed and the output image to be presented after processing will be maintained at the same level. Unfortunately, as output resolution levels increase to higher and higher output levels in terms of dots per inch printed for example, a prohibitive level of data storage and computation is required. For example, according to current product line projections, binary printer resolution goes as high as 2500 dots per inch (dpi) in the case of photo-typesetter systems. This level of output resolution effectively overwhelms current printer data processing system capabilities.
However, since it is well known that at normal viewing distances (e.g., 10-12 inches), continuous tone images appear essentially faultless in terms of spatial resolution on the order of approximately 150 dpi. Accordingly, despite the higher resolution levels available at the printer output, there is an opportunity to control data processing loads by accepting a lower, nontheless acceptable resolution level.
This has been accomplished to a limited extent within the current prior art. For example, a digitized computer image can be published with a typical binary-raster printer or display device at a resolution level set by the image initially acquired through a scanner as an input raster con-tone image on the order of 8-bits con-tone. The output printer device exhibits the same spatial resolution capabilities as the input scanner. In processing the input digitized con-tone image at the given resolution level, the data is examined line by line, and pixel by pixel. With each pixel, the question is asked, whether its gray value is closer to black (which is say 0 on the 8 bit con-tone), or closer to white (i.e., 255 on the 8 bit con-tone scale. Each input pixel data item is thus represented at a corresponding binary output pixel level set at either black or white, for example. Then, a difference value between the input value for the pixel and its output level set to either black (0) or white (255) is established. This is the error which is divided up, distributed and added to adjacent pixels yet to be processed. Thus, a light-gray input would be printed white, and the adjacent pixels would be diminished by the error to darker grays, which if sufficiently high would result in printing a black output when their turn to be processed comes. According to this prior art approach, an area of input pixels having a particular shade of gray will result in a mix of black and white output pixels, which if viewed at an appropriate distance would present a desired accurate appearance of that same shade of gray. The method has, according to the prior art, additionally been extended beyond merely application to binary printers to printers with a number of levels of gray and to color printers as well, these being effective for printing dots using a restricted number of primary colors, but nonetheless achieving the desired appearance of continuous variation in value, hue and chrome at appropriate viewing distances. However, various undesirable artifacts nonetheless appear at the printer output when this technique is used, depending on how the error is divided up and distributed. These undesirable effects may take on the appearance of lines or worms, for example, which were clearly not present at the input con-tone image.
It is accordingly an object of the invention to enable the printing, reproduction, and display of physical images from selected printers and displays which simulate the impression of selected physical input con-tone images without suffering the disadvantageous data processing loads associated with printer systems handling large input pixel data streams, while at the same time minimizing visual artifacts which commonly appear in the output presentation printer or image display.